examples/cogl-texture-pixmap-x11: Add a --stereo command line option
examples/cogl-texture-pixmap-x11: Add a --stereo command line option

If --stereo is passed, then the texture pixmap is created as a stereo
texture pixmap, and also, if passed in conjunction with --gears,
glxgears is also run with the -stereo option.

Reviewed-by: Robert Bragg <faf498254fde8ea06a623b1954d6cdfdde02c95b@intel.com>

djdeath/cogl,djdeath/cogl,djdeath/cogl